The present invention relates to magnetic chucks and more particularly to permanent magnetic chucks of the type in which a top plate and a shiftable pack each comprise a plurality of major pole pieces alternating with permanent magnets, the top plate and the pack having equal pole pitches so that the pack can be shifted between a first position in which its magnets aid the top plate and a second position in which its magnets oppose the top plate.
In permanent magnet chucks of this type known heretofore, the character of the top plate has been uniform across its entire surface, the pole pitch being chosen as a compromise between a coarse pitch which was most suitable for large workpieces and a finer pitch which provided better holding of thin workpieces. Typically, such chucks are rectangular with the pole pieces extending transversely, i.e. across the width of the chuck and with the shifting of the pack occurring lengthwise. In some cases a floating pole was provided between major poles to somewhat improve the holding of thin workpieces.
Among the several objects of the present invention may be noted the provision of a magnetic chuck of the permanent magnet type which provide greatly improved holding power for thin workpieces without significantly sacrificing the holding power available for most thick workpieces; the provision of such a chuck which is suitable for holding a wide variety of workpieces such as may be encountered in a typical machine shop toolroom; the provision of such a chuck which preserves the known advantages of permanent magnet chucks in which releasing of a workpiece is obtained by merely shifting a pack in relation to a top plate, the pitch of the top plate and the pack being equal; the provision of such a magnetic chuck which is highly reliable and which is of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.